1. Field of the Invention
The present invention relates to a polishing method of a disk-shaped substrate such as a glass substrate for a magnetic recoding medium, a polishing apparatus and the like.
2. Description of the Related Art
In recent years, the production of disk substrates as disk-shaped substrates has been activated, under increased demands as recording media. As a magnetic disk substrate that is one of the disk substrates, an aluminum substrate and a glass substrate are used widely. The aluminum substrate is characterized by its high processability and low cost, meanwhile the glass substrate is characterized by its excellent strength, surface smoothness and flatness. In particular, requirements for compact size and high density of disk substrates recently have become extremely high, and the glass substrate of which surface roughness is small and that enables high density has attracted a lot of attention.
Various improvements have been made for manufacturing method of such the magnetic disk-shaped substrate. For example, there is a related art in which an edge face of the glass substrate is polished by brush polishing in order to address a problem that a projection portion is formed on the magnetic disk surface by particles on the glass substrate, and the projection portion becomes thermal asperity and fluctuates a resistance value of a magnetoresistive head (See Patent Document 1, for example). In Patent Document 1, by brush polishing using slurry, edge face portions of the glass substrate (angular portions, side surfaces and chamfered portions) are polished while the glass substrate is rotated so that the angular portion is made into a curved surface and surface roughness falls within a predetermined range.
Also, as another Patent Document, with the purpose of efficient outer circumferential surface work of a glass disk, lowered facility costs and improved yield, there is a related art in which a polishing brush rotating with respect to piled glass disks is reciprocated and oscillated vertically (See Patent Document 2, for example). In this Patent Document 2, work efficiency is further improved by polishing plural sets of piled glass disks with plural polishing brushes.
[Patent Document 1]    Japanese Unexamined Patent Application Publication No. 10-154321.
[Patent Document 2]    Japanese Unexamined Patent Application Publication No. 11-28649.
Here, polishing using a brush is carried out in a process after grinding processing in general. Therefore, numerous grinding marks are present on the edge face of the glass substrate, and the circumference of the numerous grinding marks is preferably polished to smooth the surface. At this time, if a brush with a bristle made of polyamide resin and the like is used for polishing, polishing ability of the brush is low and deep grinding marks may not be smoothed only by the polishing ability of slurry. Particularly, in order to make an angular portion on the edge face into a curved surface, a diameter of the brush tip end is required to be reduced so that the brush enters the chamfered portion, but when a thin brush bristle is used in order to reduce the diameter of the brush tip end, the brush will lose stiffness and the polishing ability will be lowered.
The present invention is made in order to address the above problems and has an object to improve polishing performance in a polishing process by a brush and to smooth marks such as grinding marks.